Elastomers may be used in a variety of processing and manufacturing operations which demand resilience and resistance to high temperatures and chemical attack. For example, elastomeric seals may be used in chemical processing and petroleum refining equipment, such as pump housings, reactors, mixers, compressor casings, valves and other equipment. Elastomeric seals are also used in the semiconductor manufacturing industry, often in the presence of aggressive chemicals and specialty gases. In the nuclear power industry, such seals may be used in check valves and pressure relief valves where superior reliability is required. In analytic and process instrument applications, such as liquid chromatography, microcontamination is often unacceptable and seals or gaskets must possess superior resistance to degradation when subjected to chemical attack. Chemical and temperature resistant elastomeric seals are also required in the aircraft and aerospace industries, chemical transport industries, and paint and coating operations, to name a few.
Such seals may be located within complex machinery and process systems, making replacement of failed seals a costly and arduous task. Most elastomeric seal failures are caused by thermal aging and fluid attack. The polymers may degrade by oxidation, chemical attack, breaking of the polymer chain, etc. Swelling may cause the seal to expand out of its retaining grooves and cause leaks in a system. Generally, higher temperatures increase the deteriorative effect of chemicals on polymers. Thermal aging, including temperatures caused by peak loading conditions of equipment, may cause elastomers to become hard and brittle, decreasing the ability of seals to conform to irregular surfaces. In addition, there is evidence that the presence of oxygen may deteriorate some elastomers subjected to temperatures greater than 200.degree. F.
The prior art discloses a variety of modified fluoroelastomeric materials having various improved physical properties. For example, U.S. Pat. No. 4,387,168 discloses an adhesive composition comprising a low molecular weight fluorine-containing elastomer and fibrillated polytetrafluoroethylene (PTFE) which has improved chemical and weather resistance. Examples of suitable elastomers include copolymers of vinylidene fluoride and at least one fluorine-containing ethylenically unsaturated monomer, such as tetrafluoroethylene (TFE), trifluorochloroethylene, trifluoroethylene, hexafluoropropylene (HFP), pentafluoropropylene, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether). A conventional filler such as silica may be included as an additive in the composition.
The use of a fibrillated filler such as fibrillated PTFE is not desirable, however, because the fibrillated filler is deleterious to processibility, hindering milling, mixing, extrusion, and mold flow. Articles formed from elastomeric compositions including fibrillated fillers may be stiff and have high modulus and die swell values. Articles produced by extruding compositions containing a fibrillating filler typically have poor surface characteristics, such as gaps, cracks, etc. The preform cross-section is usually non-uniform causing uneven surfaces in the final article. Smooth surface characteristics are particularly important for articles to be used as seals to prevent leakage around the seal area.
U.S. Pat. No. 5,057,345 discloses a polymer blend comprising (A) a fluorinated ethylene-propylene copolymer and (B) a fluoroelastomer. The fluoroelastomer comprises a block copolymer having at least one elastomeric segment comprising TFE, vinylidene fluoride and HFP repeating units and at least one nonelastomeric segment comprising TFE and ethylene repeating units. The advantages of these blends include high tensile strength or elongation, low modulus, increased flexibility, improved stress-crack resistance, stress-induced crystallization and/or optical clarity.
U.S. Pat. No. 4,952,630 discloses a dispersion-process-produced, non-melt-processible, particulate, core-shell, TFE copolymer which may be compositioned with an elastomer or plastic. The PTFE resins comprise recurring units of TFE and modifying recurring units of at least one ethylenically unsaturated comonomer that is copolymerizable with the TFE in a dispersion process. Preferred comonomers include HFP and perfluoro(alkyl vinyl ethers). The comonomer may be present throughout the copolymer article, if desired, for example in the core as well as in the shell. The TFE copolymer may be incorporated in an elastomeric or a plastic organic resin. The resulting elastomeric blends have improved tear strength and abrasion resistance. The resulting plastic blends have improved extrusion properties, rate, abrasion resistance, flame resistance and less melt-swell. The elastomer matrix may be any elastomer including, but not limited to, vinylidene fluoride copolymers. The elastomeric or plastic matrix can contain fillers, such as reinforcing agents or fire retarding agents.
R. Morgan et al., "Reinforcement of Elastomers With Fluoroplastic Additives", E. I. du Pont de Nemours & Co. presentation to the Energy Rubber Group in Arlington, Tex. (Jan. 16, 1991) (originally presented at a meeting of the Rubber Division of the American Chemical Society in Washington, D.C. (Oct. 12, 1990)) discloses that elastomers compositioned with a high molecular weight TFE/HFP fluoroplastic micropowder (Teflon.RTM. MP1500, available from E. I. du Pont de Nemours of Wilmington, Del.) improves the tear strength and abrasion resistance and reduces the coefficients of friction of elastomers. The micropowder forms short fibers, ribbons or platelets when compositioned with sufficient shear into elastomers.
U.S. Pat. No. 4,713,418 discloses blending a thermoplastic TFE copolymer with a fluoroelastomer at a temperature which is sufficiently high to melt the thermoplastic TFE copolymer. Suitable fluoroelastomers include copolymers of 52 to 79.9 mole percent TFE, 20 to 45 mole percent perfluoro(alkyl vinyl ethers) and a third comonomer which can act as a crosslink site. Carbon black may be added to the blend to improve the strength of articles formed from the blend.
Such prior art fluoroelastomeric materials may deteriorate when exposed to heat and harsh chemical environments, such as hot aromatics, oils, heat transfer fluids, amines, corrosive acids and steam, typically found in the chemical processing and other high technology industries. For example, fluoroelastomeric seals are vulnerable to attack from methyl ethyl ketone ("MEK"), diethylamine, aqueous ammonia, glacial acetic acid and other agents.
There is a long-felt need in the art for a process or method for making fluoroelastomeric compositions and seals reinforced with a non-fibrillating filler other than carbon black which may be used to form articles, such as seals, which resist deterioration when exposed to harsh chemical environments and function over a broad spectrum of temperatures from well below freezing to in excess of 450.degree. F. without deteriorating, creeping or flowing. Preferably, such compositions and seals would combine the sealing ability of elastomers with superior chemical resistance over a wide range of temperatures.